I 



' - 52° 28' 



^ LIEUT. -GENERAL SABINE ON TERBESTEIAL MAGNETISM. 



were reversed and the needle and circle used in the eight ordinary positions, whenever 

 opportunities presented themselves for the comparison either on land or on ice. The 

 needles were distinguished as R, F, 5 in the ' Erebus,' and F, C, B in the ' Terror.' 



The dips were observed either "Direct" or by the aid of "Deflectors;" the inten- 

 sities of the Force occasionally by weights and occasionally by deflectors ; on land always 

 by both methods, and at sea occasionally so. In the * Erebus' and ' Terror,' in which the 

 whole, or nearly the whole of the disturbance arising from the ship's iron was caused by 

 induced magnetism, the deviation of the declination in the southern hemisphere was, as 

 we have seen, a maximum to the East when the ship's head was to the West, and to 

 the West when the head was to the East, passing through its zero when the ship's head 

 was either north or south, or nearly so. In the Inclination and Total Force, on the 

 other hand, the deviation (always speaking of the phenomena in the Southern Hemi- 

 sphere) was a maximum when the ship's head was approximately either north or south, 

 and passed through its zero as the ship's head was directed towards the east or towards 

 the west. The dip of the south end of the magnet was least and the south polar force 

 greatest when the ship's head was to the south, and the south dip greatest and the south 

 polar force least when the head was to the north. 



Corrections to he applied for the Deviations of the Inclination. 

 1. In the ' Erebus.' — The values of the constants c and d, employed in the correc- 

 tion of the deviations of the Inclination observed in this portion of the Survey, have 

 been derived from the results obtained at the usual place of observation on board, with 

 the ship's head on the difierent points of the compass, by the well-known process of 

 " swinging the ship;" such results were obtained at Port Louis on the 17th of August, 



1842, and at Simon's Bay on the 20th of April, 1843, the interval between those dates 

 comprising the whole of the third year's survey. The observations made at Port Louis on 

 the 17th of August, 1842, are printed in No. VI. of these Contributions (Philosophical 

 Transactions, 1844, Art. VII., p. 168). Those at Simon's Bay on the 20th of April, 



1843, will be found in page 459 of the present Number. 



At Port Louis the mean of the results on the sixteen points was —52° 24'; and the 

 Inclination observed on shore with the same instrument with the face of the circle 

 also towards the east, and the needle observed "direct" and with "deflectors," was 



At Simon's Bay the mean on the sixteen points was —53° 42', and the Inclination 

 observed on shore with the same instrument, face east and needle " direct " and with 

 "deflector S," was —53° 87'-5. In both cases the mean of the results on the sixteen 

 points has been taken as the standard of reference for assigning the deviation on the 

 several points. 



In the subjoined Table (Table III.), column 2 contains the dips observed at Port Louis 

 on the different points of the compass specified in column 1 ; and column 3 the deviation 

 on each point from the mean of the sixteen points stated at the foot of the column. 



